The present invention relates to a use of HF Radar. It particularly relates to HF Radar installations consisting of arrays of receiving and transmitting antennas configured to produce near-vertical incidence of radiation paths to and from the ionosphere overhead to illuminate a land or sea area near to the antennas with horizontally polarised radiation, HF Radar being normally specified to be in the frequency range of 2-30 MHz.
As depicted in FIG. 1, whereas microwave radar is generally limited to line-of-sight surveillance, HF Surfacewave Radar allows xe2x80x98over-the-horizonxe2x80x99 surveillance to be made. This is due to electromagnetic waves at HF having the important property of being refracted by the ionosphere so as to return to earth. As depicted in FIG. 2, HF Skywave Radar also allows xe2x80x98over-the-horizonxe2x80x99 surveillance but with the constraint that detections cannot be made at distances less than some 600 km, the so-called xe2x80x9cskip distancexe2x80x9d.
It is the practice to transmit vertically polarised signals in HF Surfacewave Radar to achieve coupling to the conducting surface of the sea. Vertical polarisation is also used for HF Skywave radar for ease of antenna construction.
Where the radar target is a helicopter, the Skywave form of HF Radar is known to obtain detections due to the downward signal from the ionosphere being reflected from the rotating blades (the downward signal is horizontally polarised due to the refraction mechanisms at the ionosphere). The reflections from the multiple blade rotor rotation result in a characteristic, identifiable modulation of the HF Radar signal.
Where detections by microwave radar of targets such as helicopters, other aircraft or surface vessels are to be made at short distances from the radar, typically 20 km to 150 km, intervening topography such as mountainous terrain, may prohibit xe2x80x98above-the-horizonxe2x80x99 radar detection.
Lack of sea in the foreground could prohibit HF Surfacewave xe2x80x98below-horizonxe2x80x99 detection, also its obligatory vertical polarisation is not in the horizontal plane which is required for optimal reflection from the rotating near horizontal rotor blades of helicopters; the minimum skip-distance criterion will prohibit detection by conventional HF Skywave transmission. In these cases recourse could be made to surveillance over-flights of the target area by rotary or fixed wing aircraft or by satellite borne sensors.
For operational reasons, or if the target is stealthy to microwave detection, these detection methods may be limited in application. In particular, a slow-moving, low-flying helicopter would be difficult to detect when shielded by terrain or where the background produces radar clutter.
In order to produce a response from the rotor blades of a low-flying helicopter with horizontally polarised energy in the HF radio band, the mode of transmission and reception called Near-Vertical Incidence Skywave (NVIS) will be utilised, where horizontally polarised radiation is launched from a suitably configured HF transmitting antenna array in directions lying within an inverted cone of some 30xc2x0-apex angle. By suitable choices of radiated frequency within the HF band, downward refraction can be achieved over a significant part of the 24-hour diurnal sun cycle (the sun""s radiation causes the necessary ionisation for producing this refraction).
The downward-travelling signal illuminates the earth""s surface together with targets including ships and aircraft moving over it. Back-scattered returns from these will travel upwards in a similar path direction which will allow a further refraction at the ionosphere causing the signal to travel down again to the vicinity of the transmitting site. Normally, near to the transmitting antenna array is located a horizontally polarised upwardly-directed receiving antenna array for intercepting the returns from the illuminated target area.
The present invention provides an arrangement whereby the disadvantages of the prior art are overcome by obtaining returns from targets which may be hidden from normal sensors by mountainous terrain, steep-sided fjords, by below-the-horizon limitations, or where the target may be stealthy (with reduced visibility) to microwave radar.
According to the present invention there is provided an HF radar system comprising a transmitting system, a receiving system, a signal processing system and a frequency management/ionospheric sounding system;
the transmitting system comprising a transmitting antenna array configured to transmit a beam in a near vertical direction and a transmitting device arranged to drive the transmitting antenna array at frequencies suitable for downward refraction by the ionosphere;
the receiving system comprising a receiving antenna array configured to receive returning signals from a target area returning to the receiving antenna array via refraction at the ionosphere;
the signal processing system comprising a digital data processing system;
the frequency management/sounding system comprising a cooperating transmitting system and receiving system sending HF signals to the ionosphere and analysing the returning signals.
The receiving system includes means to discriminate a returning signal from a helicopter.
Alternatively, according to the present invention there is provided an HF radar system comprising a duplexed antenna array, a transmitting system, a receiving system, a signal processing system and a frequency management/ionospheric sounding system;
the duplexed antenna array comprising a composite directional transmitting and receiving antenna array connected to a duplexer and configured to transmit a beam in a near vertical direction and to receive returning signals from a target area returning to the composite transmitting and receiving antenna array via refraction at the ionosphere;
the transmitting system comprising a transmitting device being connected via the duplexer to and arranged to drive the composite directional transmitting and receiving antenna array at frequencies suitable for downward refraction by the ionosphere;
the receiving system being connected to the duplexer and being configured to receive returning signals from the target area returning to the receiving antenna array via refraction at the ionosphere;
the signal processing system comprising a digital data processing system;
the frequency management/sounding system comprising a cooperating transmitting system and receiving system sending HF signals to the ionosphere and analysing the returning signals.